Drying coated substrates, such as webs, typically requires heating the coated substrate to cause liquid to evaporate from the coating. The evaporated liquid is then removed. In typical conventional impingement drying systems for coated substrates, one or two-sided impingement dryer technology is utilized to impinge air to one or both sides of a moving substrate. In such conventional impingement dryer systems, air supports and heats the substrate and can supply heat to both the coated and non-coated sides of the substrate. For a detailed discussion of conventional drying technology see E. Cohen and E. Gutoff, Modern Coating and Drying Technology (VCH publishers Inc., 1992). In a gap drying system, such as taught in the Huelsman et al. U.S. Pat. No. 5,581,905 and the Huelsman et al. U.S. Pat. No. 5,694,701, which are herein incorporated by reference, a coated substrate, such as a web, typically moves through the gap drying system without contacting solid surfaces. In one gap drying system configuration, heat is supplied to the backside of the moving web to evaporate solvent and a chilled platen is disposed above the moving web to remove the solvent by condensation. The gap drying system provides for solvent recovery, reduced solvent emissions to the environment, and a controlled and relatively inexpensive drying system. In the gap drying system, the web typically is transported through the drying system supported by a fluid, such as air, which avoids scratches on the web.
As is the case for impingement dryer systems, previous systems for conveying a moving web without contacting the web typically employ air jet nozzles which impinge an air jet against the web. Most of the heat is typically transferred to the back side of the web by convection because of the high velocity of air flow from the air jet nozzles. Many impingement dryer systems can also transfer heat to the front side of the web. In an impingement dryer system, the air flow is highly nonuniform, which leads to a non-uniform heat transfer coefficient. The heat transfer coefficient is relatively large in the region close to the airjet nozzle which is referred to as the impingement zone. The heat transfer coefficient is relatively low in the region far from the air jet nozzle where the air velocity is significantly smaller and tangential to the surface. The non-uniform heat transfer coefficient can lead to drying defects. In addition, it is difficult to uniformly control the amount of energy supplied to the backside of the web because the air flow is turbulent and complex. The actual effect of operating parameters on the drying rate can usually only be determined after extensive trial and error experimentation.
One method of obtaining a more uniform heat transfer coefficient to the web is to supply energy from a heated platen to the backside of the web by conduction through a fluid layer between the heated platen and the moving web. The amount of energy supplied to the backside of the web is a function of the heated platen temperature and thickness of the fluid layer between the heated platen and the moving web. In this situation, the heat transfer coefficient is inversely proportional to the distance between the heated platen and the moving web. Therefore, in order to obtain large heat transfer coefficients which are comparable to those obtained by air impingement drying systems, the distance between the moving web and the heated platen needs to be very small. In many applications, the web must not touch the heated platen to prevent scratches from occurring in the web. However, in some applications a degree of contact between the web and the heated platen is not detrimental to a product produced from the web coated material and high heat transfer rates are required or desired. In these other types of applications, it is advantageous to have the capability of metering away a sufficient amount of the fluid layer to enable the web to contact the heated platen.
In certain gap drying system applications, the heat transfer from the heated platen through the fluid layer to the moving web becomes non-uniform. In such an application, the non-uniform heat transfer from the heated platen to the moving web causes non-uniform drying of the coating on the substrate which produces drying patterns on the dried coated web.
For reasons stated above and for other reasons presented in greater detail in the Description of the Preferred Embodiments section of the present specification, a drying system is desired which provides more uniform heat transfer to the moving coated substrate and more uniform drying of the coating on the substrate to thereby reduce the incidence of drying patterns on the coated substrate caused by non-uniform heat transfer. In addition, there is a need for a drying system where the heat transfer and drying rates are more easily controlled.